where to configure DNS TTL minimal values ?

some time ago there was solved issue in butracker about limits for minimal values which user can input to DNS (eg not less than 3600)
The issue was fixed and i asked once where the limits can be set.
I got answer that somewhere in the script.

I cannot find the issue or the answer. Can anybody advice me where i can set minimal limits which user can set for DNS? Users are settng nosenses like 60 second etc..